This invention relates to the field of magneto-optic recording. More particularly, it relates to improvements in magneto-optic playback apparatus.
In magneto-optic recording systems, information is recorded on a magnetic recording element by selectively flipping the magnetic domains along a desired data track in accordance with information to be recorded. To recover this information, a beam of linearly polarized radiation is scanned along the data track. Depending on the vertical orientation of the irradiated magnetic domains, the state of polarization of the incident beam is altered, such alteration being manifested by a slight rotation (e.g. about a degree or so) of the plane of polarization of the incident beam. This small change in polarization is detectable, for example, by passing the modified beam through a polarization analyzer having its axis set at 90.degree..+-.the slight rotation angle relative to the plane of polarization of the incident beam, and monitoring the variations in intensity of the beam transmitted by the analyzer. Alternatively, and more conventionally, the polarization change is detected by passing the modified beam through a conventional polarizing beam splitter having its axis oriented at 45.degree. relative to the direction of polarization of the playback beam, and positioning a detector in each of the split beams. The respective outputs of the detectors are subtracted to derive the data signal. This well-known differential detection scheme affords the advantage of common-mode rejection of certain noise sources.
In any magneto-optic system, it is necessary to maintain the read beam in sharp focus on the recording element. Moreover, in most systems, the beam's lateral position must be precisely controlled to assure that a desired data track is being followed. To meet these requirements, conventional magneto-optic recording systems incorporate independent sensor systems to produce focus and tracking error signals based on the quality of focus and the tracking accuracy. These error signals can be derived, for example, from the four output signals produced by a so-called "quad-cell", i.e. four independent and equi-sized photodetectors symmetrically arranged to cover the four quandrants of the beam. Prior to being detected by the quad-cell, the beam is astigmatized, usually by passing it through a cylindrical or toric lens. The focus and tracking error signals are derived by summing and differencing the quad-cell outputs in a well-known manner as described, for example, by the disclosure of U.S. Pat. No. 4,410,969 (Maeda). Many other detection schemes (e.g. knife-edge, bi-prism, ring-lens, etc.) are also known for producing these error signals.
In conventional magneto-optic playback systems, it is known to use a plurality of partially reflective/partially transmitting beam-splitters to produce the required number of beams for differential detection and focus/tracking error detection. For example, referring to the prior art system shown in FIG. 1, three beam-splitters may be used. Here, a read beam B, as produced by a laser L, is passed through BS1 which polarizes the beam and is focused onto a magneto-optic recording element R by an objective lens O. Upon being reflected from and modified by the recorded information on the recording element, the beam is split into two beams B' and B" by beam-splitters BS1 and BS2. Beam B' is astigmatized by a toric lens CL and focused onto a quad-cell QC. After suitable signal processing by a control circuit C, focus and tracking error signals, E.sub.F and E.sub.T, respectively, are produced, in manner described in the aforementioned patent of Maeda. Beam B" is split again by a polarizing-beam-splitter PBS, and the output thereof is differentially detected by a pair of photodetectors D, D' and a differential amplifier DA. The output of E.sub.s of the differential amplifier represents the recorded information, i.e., the data signal.
The need for three separate beam splitters in magneto-optic systems of the type shown in FIG. 1 presents problems for commercial applications where size, weight, and cost are to be minimized.